The present invention relates to a semiconductor device, and more particularly to a semiconductor device having thickness different gate oxide films and a method of forming the same.
One of the most important issues for development of the semiconductor devices is how to improve the reliability of the gate oxide films and also realize a high controllability in thickness of the gate oxide films. The thickness of the gate oxide film of the field effect transistor is different between in the logic device and the peripheral device. The most important issue is how to realize a high controllability information of the gate oxide films different in thickness over a single semiconductor substrate.
It has been known in the art to which the invention pertains that two gate oxide films having different thicknesses are formed by two different oxidation processes. FIGS. 1A through 1C are fragmentary cross sectional elevation views illustrative of the conventional method of forming the two gate oxide films having different thicknesses of the two field effect transistors on a single semiconductor substrate.
With reference to FIG. 1A, Field oxide films 11 are selectively formed on a surface of a silicon substrate 10 to define two device regions "A" and "B" as active regions of the silicon substrate 10. Gate oxide films 19 are formed on the two device regions as active regions of the silicon substrate 10.
With reference to FIG. 1B, a photo-resist film 20 is selectively formed on the device region "B" of the silicon substrate 10. The gate oxide film 19 but only on the device region "A" is subjected to a wet etching by use of a hydrofluoric acid based etchant to remove the gate oxide film 19 from the device region "A".
With reference to FIG. 1C, the photo-resist film 20 is then removed and a cleaning process is carried out before a heat treatment is carried out to cause a second oxidation, whereby a thin gate oxide film 23 is formed on the device region "A" of the silicon substrate 10 whilst the gate oxide film 19 on the device region "B" is made thicker to form a thick gate oxide film 22 on the device region "B". As a result, the thickness different two oxide films are formed on the two device regions.
The above conventional method is, however, engaged with the following problems. The above conventional method needs the process for applying a photo-resist on the gate oxide film 19 on the device region "B" for subsequent wet etching process to remove the other gate oxide film 19 on the device region "A". The photo-resist contains various contaminants such as irons and organic substances. After the photo-resist is applied on the gate oxide film 19 on the device region "B", then the various contaminants such as irons and organic substances may be introduced into the gate oxide film 19 on the device region "B", whereby the insulating property or reliability of the gate oxide film 19 on the device region "B" is deteriorated.
Further, after the photo-resist has been removed from the device region "B", then the cleaning process is carried out by use of a small amount of the etchant which causes a slight etching to the surface region of the gate oxide film 19 on the device region "B", whereby the thickness of the gate oxide film 19 on the device region "B" is slightly reduced. It is difficult to control the reduction in thickness of the gate oxide film 19 on the device region "B", for which reason it is difficult to control the reduction in thickness of the thick gate oxide film 22 on the device region "B".
Furthermore, the thermal oxidation process for forming the oxide film allows a diffusion of oxidation seeds in the film to provide a large influence to an oxidation rate, for which reason it is difficult to realize a high controllability in thickness of the oxide film. This thermal oxidation process is largely different from the deposition process such as a chemical vapor deposition for forming the oxide film
Another method of forming the thickness different two gate oxide films over a single semiconductor substrate has been known, wherein an ion-implantation is utilized. Nitrogen atoms have been introduced into a silicon substrate before oxidation is made to the surface of the silicon substrate, whereby the oxidation rate is largely suppressed by the implanted nitrogen atoms. For example, the nitrogen atoms have been selectively implanted into the device region "A" only, before the thermal oxidation is made to both surfaces of the nitrogen introduced device regions "A" and the nitrogen free device region "B", whereby the oxidation rate on the nitrogen introduced device regions "A" is slower than the oxidation rate on the nitrogen free device region "B" in order to differentiate the thicknesses of the two gate oxide films on the nitrogen introduced device regions "A" and the nitrogen free device region "B". This second conventional method needs a single oxidation process and a single ion-implantation process.
FIG. 2 is a diagram illustrative of variations in thickness of a gate oxide film over oxidation time under various conditions of dose of nitrogen ions into a silicon substrate, thereby illustrating a relationship of a growth rate of the gate oxide film to the nitrogen ion dose. If no nitrogen ion is introduced into the silicon substrate, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 8 nanometers. If nitrogen ions are introduced into the silicon substrate at a dose of 5E13, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 6.8 nanometers. If nitrogen ions are introduced into the silicon substrate at a dose of 1E14, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 5.2 nanometers. If nitrogen ions are introduced into the silicon substrate at a dose of 5E14, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 3.6 nanometers. If nitrogen ions are introduced into the silicon substrate at a dose of 1E15, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 2.8 nanometers. The increase in dose of the nitrogen atoms into the silicon substrate reduces the oxidation rate of the oxide film.
FIG. 3 is a diagram illustrative of variations in flat-band voltage of a field effect transistor having a gate oxide film formed on an nitrogen containing surface of a silicon substrate under various conditions of dose of nitrogen ions into the silicon substrate. Increase of ion-implanted nitrogen atoms causes the increase of the fixed charges and also drop in electron mobility of the transistors. If the nitrogen ions are introduced into the silicon substrate at a dose of not less than 7E14 atoms/cm.sup.2, the performance of the transistor having a thinner gate oxide film is dropped below the acceptable performance range. Namely, it is unavailable to carry out the ion-implantation of nitrogen at a high dose of not less than 7E14 atoms/cm.sup.2.
An alternate available method for controlling the thickness of the gate oxide film or suppressing the oxidation rate of the gate oxide film is to carry out an ion-implantation of halogen atoms into the silicon substrate in place of the nitrogen atoms. FIG. 4 is a diagram illustrative of variations in thickness of a gate oxide film over oxidation time under various conditions of dose of halogen atoms into a silicon substrate, thereby illustrating a relationship of a growth rate of the gate oxide film to the halogen atom dose. If no halogen atom is introduced into the silicon substrate, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 8 nanometers. If halogen atoms are introduced into the silicon substrate at a dose of 5E13, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 9 nanometers. If halogen atoms are introduced into the silicon substrate at a dose of 2E14, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 11 nanometers. If halogen atoms are introduced into the silicon substrate at a dose of 7E14, then a thermal oxidation for 30 minutes results in a formation of an oxide film having a thickness of 13 nanometers. The increase in dose of the halogen atoms into the silicon substrate increases the oxidation rate of the oxide film. This method doping the halogen atoms into the silicon substrate is also engaged with the following problems.
FIG. 5 is a diagram illustrative of variations in density of the interface state of the gate oxide film formed on a fluorine introduced silicon substrate surface by a thermal oxidation in a moisture atmosphere. The density of the interface state becomes minimum at the dose of fluorine atoms of about 1E14 atoms/cm.sup.2. However, the increase in the dose of fluorine atoms from about 1E14 atoms/cm.sup.2 also increases the density of the interface state, whereby the performances of the transistor are deteriorated. The dose of fluorine atoms of about 1E14 atoms/cm.sup.2 relaxes the strain of the interface between silicon and silicon oxide. However, the higher dose than about 1E14 atoms/cm.sup.2 results in no relaxation in the strain of the interface between silicon and silicon oxide. In order to realize a large difference in thickness of the gate oxide films, for example, 3 nanometers, it is necessary to introduce fluorine atoms at a dose of not less than 5E14 atoms/cm.sup.2. The introduction of fluorine atoms at a dose of not less than 5E14 atoms/cm.sup.2 results in a higher density of the interface state than the acceptable density range.
In the above circumstances, it had been required to develop a novel structure of two thickness different gate oxide films over a single silicon substrate free from the above problems and a method of forming the same.